Morphology, composition and dissolution of chromite in the Goro lateritic nickel deposit, New Caledonia: Insight into ophiolite and laterite genesis
Martin Wells, Erick Ramanaïdou, Zakaria Quadir, Malcolm P. Roberts, Julien Bourdet, Michael Verrall
Abstract
The morphological and chemical composition of chromite in the partially-serpentinized harzburgite and oxide zone of two laterite profiles at the Goro lateritic nickel deposit, New Caledonia, were investigated to: (1) establish the melt conditions at the mantle-crust boundary during ophiolite formation, and (2) characterize the weathering of chromite and its role in the goethite maturation model in formation of the oxide zone at the Goro Ni-deposit. Chromite in the Goro harzburgite, despite being highly fractured, is relatively fresh. Dendritic branching of skeletal outgrowths from poikiloblastic chromite, encapsulate partially serpentinised harzburgite and is indicative of a parent melt under changing cooling conditions, varying from supersaturated to undersaturated with localised melt mixing. Compositionally, the refractory nature (Cr#>0.6) of chromite, low (predominantly < 0.05 wt%) TiO2 content and high logFe2+:Fe3+ ratio values of 14–16 indicate a highly depleted and reduced peridotite melt with boninite-type affinity of suprasubduction provenance. Furthermore, the high Cr# of Goro chromite may define a compositional end-member along a continuum of Al2O3-Cr2O3 exchange with a relatively consistent (i.e., 25–35 wt%) FeO content, from the Goro deposit (Cr > Al) in the south-east through to high-Al chromite (Cr < Al) at Poum in the north-west of New Caledonia. In the oxide horizon, extensive surface etching and development of solution channels throughout chromite grains, supports both their abiotic and biogenically mediated dissolution. Abiotic dissolution is evidenced by the tessellation of surface etching features and epitactic oxidation of chromite to Cr-substituted hematite. Extensive development of meandering dissolution channels through the grain interior and the precipitation of locally elevated Mn (to 3–4 wt%) that juxtapose areas depleted in Cr but elevated in Al, Mg and Fe support the inferred role of Mn-oxidizing bacteria in the preferential dissolution of Cr from chromite. Preferential leaching of Cr from chromite, whether by abiotic and/or biogenic means, feeds into the mineralogical evolution of the oxide zone at Goro and supports an alternative crystallisation model via a ‘green-rust-like’ precursor for the formation of poly-metallic-substituted goethite.